1. Field of the Invention
The present invention relates to a curve approach control apparatus which calculates, based on an allowable lateral acceleration, an allowable approach speed at which a vehicle negotiates a curve in front and, based on the allowable approach the curve.
2. Description of the Prior Art
Many curve approach control apparatus have been proposed which use road map data in a navigation device to detect an overspeed state of a vehicle with respect to a curve in front and activate an alarm or deceleration control.
For example, Japanese Patent Unexamined Publication No. Hei. 10-269499 (JP-A-10-269499) discloses a technology which judges presence or absence of a curve in front of the vehicle based on various information from a navigation device, calculates a radius of curvature of the curve, detects a driving state (sportiness level) of a driver and, based on the driving state information, sets an allowable lateral acceleration at which the vehicle can trace the curve. Then, based on the radius of curvature information and the allowable lateral acceleration, the disclosed technology calculates an allowable turning speed of the vehicle in the curve (allowable approach speed) and performs control to reduce the vehicle speed down to the allowable turning speed before the vehicle enters the curve.
On a road with a high road surface friction coefficient xcexc when a limit lateral acceleration is high, a vehicle generally can turn a curve of the same radius of curvature at a higher cornering speed than on a road with a small xcexc, when the vehicle takes a corner at the limit lateral acceleration. When the limit lateral acceleration is exceeded on a road with a high xcexc and the vehicle can not trace own course and collides against some object, the damage it sustains is greater because the cornering speed is higher.
In the conventional technology described above, however, the setting of the allowable lateral acceleration does not take the above conditions of the roads with high or low xcexc into consideration and cannot be said to secure a sufficient safety.
The present invention has been accomplished under the above circumstance and provides a curve approach control apparatus which considers the condition of the road with high or low xcexc to allow the allowable lateral acceleration to be set optimumly and thereby further improve the safety of the vehicle.
To solve the above problem, according to a first aspect of the present invention, a curve approach control apparatus comprises: an allowable lateral acceleration setting means to set as an allowable lateral acceleration a lateral acceleration that a vehicle can tolerate when it negotiates a curve in front; an allowable approach speed setting means to set as an allowable of approach speed an approach speed at which the vehicle can negotiate the curve, based on at least the allowable lateral acceleration; a decision control means to estimate and judge an approach of the vehicle to the curve based on at least the allowable approach speed and execute a predetermined control; and a road surface friction coefficient estimation means to estimate a road surface friction coefficient; wherein the allowable lateral acceleration setting means corrects, according to the road surface friction coefficient, an allowable percentage of the lateral acceleration that occurs in the vehicle as it negotiates the curve and sets the allowable lateral acceleration.
According to a second aspect of the present invention, in the curve approach control apparatus shown in the first aspect, the allowable lateral acceleration setting means corrects the allowable percentage of the lateral acceleration that occurs in the vehicle as it negotiates the curve so that it decreases as the road surface friction coefficient increases.
According to a third aspect of the present invention, in the curve approach control apparatus shown in the first aspect, the allowable lateral acceleration setting means sets to a constant percentage the allowable percentage of the lateral acceleration that occurs in the vehicle as it negotiates the curve when the road surface friction coefficient is equal to or less than a preset value and, when the road surface friction coefficient is in excess of the preset value, sets the allowable lateral acceleration to a predetermined constant value.
That is, the curve approach control apparatus of the first aspect estimates the road surface friction coefficient by the road surface friction coefficient estimation means. Then, the allowable lateral acceleration setting means corrects, according to the road surface friction coefficient, the allowable percentage of the lateral acceleration that occurs in the vehicle as it negotiates the curve in front and sets the lateral acceleration that the vehicle can tolerate as the allowable lateral acceleration. The allowable approach speed setting means sets as an allowable approach speed an approach speed at which the vehicle can negotiate the curve, based on at least the allowable lateral acceleration. The decision control means estimates and judges an approach of the vehicle to the curve based on at least the allowable approach speed and executes a predetermined control.
The curve approach control apparatus of the second aspect has the allowable lateral acceleration setting means in the curve approach control apparatus of the first aspect correct the allowable percentage of the lateral acceleration that occurs in the vehicle as it negotiates the curve so that it decreases as the road surface friction coefficient increases. Hence, the curve approach control apparatus of second aspect of the present invention improves safety when the vehicle travels on a high xcexc road.
Further, the curve approach control apparatus of third aspect has the allowable lateral acceleration setting means in the curve approach control apparatus of the first aspect set to a constant percentage the allowable percentage of the lateral acceleration that occurs in the vehicle as it negotiates the curve when the road surface friction coefficient is equal to or less than a preset value. And, when the road surface friction coefficient is in excess of the preset value, set the allowable lateral acceleration to a predetermined constant value. Hence, the curve approach control apparatus of third aspect of the present invention keeps the allowable lateral acceleration at a safe value as the vehicle travels on a high xcexc road.